I am using UIActivity
in ios7. It is working fine with a few lines of code. Now i want to add instagram share button to it. Is there a way to add custom buttons
Well you have to create your own custom UIActivity, in your case for Instagram.
Have a look at this, even if the question is about on how to do it on iOS 6, the solution is exactly the same on iOS 7.
You can also have a look at this project on github, basically is a collection of custom UIActivity for the most common services Instagram included.